Space in a photograph tends to only be pleasing to people when something is travelling towards the space, or someone or something is looking into it (not a hard and fast rule, none are).

Interesting that your poll went for the space, I find the second one a bit 'middley' with the space being a bit of a distraction, but that's what its all about. Different people, different, taste, different view. Its a good job or else all photographs would be very much the same.

I think what makes the two firework photos stand out is the slight detail in the sky.